since BT5 is based off of Ubuntu (Lucid), some snooping around the 'buntu forums brought me this.
Pretty much, completely delete (apt-get remove --purge nameofapp) ettercap and then redownload the files from the ettercap section of here, choosing amd64 for 64-bit and i386 for 32-bit. (make sure you download all 3 files.) Then to install the files, cd to where they downloaded and run
Keep the "./" and swap "<.deb_file_here>" for the name of each file you downloaded.
dpkg -i ./<.deb_file_here>
Then just navigate to Internet->Ettercap or type in "ettercap -G" inside a terminal.
Hope this helps, the same thing happened to me.